1. Field of the Invention
Poly-2-pyrrolidone (also known as nylon-4) is a polymer composed of repeating structural units of the formula ##STR1##
The polymer is produced by alkaline-catalyzed polymerization of 2-pyrrolidone, preferably in the presence of a catalyst comprising the reaction product of a 5-7 membered-ring lactamate of an alkali or alkaline earth metal and carbon dioxide, e.g., the reaction product of potassium pyrrolidonate and carbon dioxide. This reaction product, which for example may be called "potassium carboxypyrrolidonate" or the "adduct of carbon dioxide and potassium pyrrolidonate", is believed to function as an initiator or activator of polymerization. In whatever manner the metal carboxylactamate may function, there is no known reason, nor any reason suggested in the literature, why the choice of a particular metallic counterion for the catalyst should effect the degree of conversion achieved in the polymerization of 2-pyrrolidone. Yet it is observed that CO.sub.2 -containing catalysts derived from sodium compounds consistently yield substantially lower conversions than CO.sub.2 -containing catalysts derived from potassium compounds under identical polymerization conditions. Generally, polypyrrolidone molecular weights determined by viscosity measurements are also substantially lower for sodium-derived catalysts than for potassium-derived catalysts under identical conditions.
There are substantial economic advantages to the use of sodium-derived catalysts over the use of potassium-derived and other catalysts under the same processing conditions. All other factors being equal, the use of sodium would be preferred for that reason alone. The only problem is that the degree of conversion of monomer to polypyrrolidone and molecular weight of polypyrrolidone achieved with sodium-derived catalysts has heretofore been so inferior as to present no real alternative to other catalyst systems.
2. Description of the Prior Art
U.S. Pat. No. 3,346,566 discloses the polymerization of 2-pyrrolidone in the presence of a catalyst produced by reacting an alkali metal alkoxide, or alkaline earth metal alkoxide, with a 5-7 membered-ring lactam and removing the alcohol released from the alkoxide either by distillation during the reaction, or by recovery of the product metal lactamate as a filterable solid after the reaction.
U.S. Pat. No. 3,721,652 discloses the polymerization of 2-pyrrolidone under conditions of alkaline catalysis in the presence of carbon dioxide. It further teaches that the alkaline catalyst, sodium pyrrolidonate, reacts with carbon dioxide to form an adduct which may be separated by precipitation from its benzene solution.
In general, the prior art teaches the interchangeable use of 2-pyrrolidone polymerization catalysts derived from Group I (alkali metal) compounds, and, in particular, does not provide a reasoned distinction between sodium- and potassium-derived catalysts in the CO.sub.2 -initiated polymerization of 2-pyrrolidone.
U.S. Pat. No. 3,835,100 (Cols. 1 and 9) teaches that alkali metal pyrrolidonate catalysts are difficult or impossible to purify and furthermore, lose catalytic activity with time.